These just appeared. What's going on?!
They seem to follow me around, but appear patchily. What have I done?
They are marking locations where hostile mobs can spawn.
It is a feature of Not Enough Items (NEI) and the default key (configurable) to toggle this on and off is F7.
Those show tiles where it is dark enough for monsters to spawn, I believe. Check in the controls to see if there is a key bound to something like that. You may have hit it by accident.